The Fn qsort and Fn heapsort functions sort an array of Fa nmemb objects, the initial member of which is pointed to by Fa base . The size of each object is specified by Fa size . Fn Mergesort behaves similarly, but requires that Fa size be greater than ``sizeof(void *) / 2''
The contents of the array Fa base are sorted in ascending order according to a comparison function pointed to by Fa compar , which requires two arguments pointing to the objects being compared.
The comparison function must return an integer less than, equal to, or greater than zero if the first argument is considered to be respectively less than, equal to, or greater than the second.
The functions Fn qsort and Fn heapsort are not stable, that is, if two members compare as equal, their order in the sorted array is undefined. The function Fn mergesort is stable.
The Fn qsort function is an implementation of C.A.R. Hoare's ``quicksort'' algorithm, a variant of partition-exchange sorting; in particular, see D.E. Knuth's Algorithm Q. Fn Qsort takes O N lg N average time. This implementation uses median selection to avoid its O N**2 worst-case behavior.
The Fn heapsort function is an implementation of J.W.J. William's ``heapsort'' algorithm, a variant of selection sorting; in particular, see D.E. Knuth's Algorithm H. Fn Heapsort takes O N lg N worst-case time. Its only advantage over Fn qsort is that it uses almost no additional memory; while Fn qsort does not allocate memory, it is implemented using recursion.
The function Fn mergesort requires additional memory of size Fa nmemb * Fa size bytes; it should be used only when space is not at a premium. Fn Mergesort is optimized for data with pre-existing order; its worst case time is O N lg N; its best case is O N.
Normally, Fn qsort is faster than Fn mergesort is faster than Fn heapsort . Memory availability and pre-existing order in the data can make this untrue.
Upon successful completion, Fn heapsort and Fn mergesort return 0. Otherwise, they return -1 and the global variable errno is set to indicate the error.